Istanbul, Turkey — Making dire predictions about Iran's nuclear programs has been a parlor game since the mid-1980s.

Jane's Defence Weekly, a well-respected military-related publication, in 1984 estimated that the Islamic Republic was two years from making an atomic bomb. By 1992, the CIA was predicting an Iranian nuclear weapon by the end of the decade. In 1995, Israel agreed. The breathless warnings have almost never stopped.

What does Iran teach us about how much a determined nation can achieve despite concerted efforts by many world powers to stop it? In recent decades, Israel, India, Pakistan, and North Korea have all in secret become nuclear weapons states.

Iran says it only wants peaceful nuclear power and that nuclear weapons are forbidden by Islam. But the UN International Atomic Energy Agency (IAEA) says a number of issues remain unresolved with Iran about possible weapons-related work.

"If Iran is now producing 20 percent enriched uranium ... frankly, they would be able to take that up to [weapons-grade] 90 percent within one month," far faster than an IAEA quarterly review could detect it, says Mr. Kile. And even with the no-notice inspections provided under the NPT's Additional Pro­tocol (which Iran no longer observes), "there are ways that a country can operationally hamper, limit, and restrict the IAEA's investigative authorities," he adds. "There really needs to be a comprehensive review of the statutory authority of the IAEA to go in and investigate where there is credible evidence to suspect" that a nonnuclear state is developing atomic weapons.

That issue is likely to rank high on the agenda at the NPT review, slated to begin May 3 in New York. But few expect IAEA Director General Yukiya Amano to be granted greater power.

Since the Security Council first imposed sanctions on Iran in 2006 and insisted that Tehran stop enrichment activities, Iran has increased the pace of enrichment and promised to expand its program. Still, experts say that US-led unilateral measures to undermine Iran's nuclear program have taken a toll.

"There is pretty broad agreement that if, at some point, Iran is determined to have the bomb, it will," says Michael Levi, a nonproliferation specialist at the Council on Foreign Relations in New York. "What we have also learned is that we are much better at severely slowing states down by interrupting their procurement efforts than we thought.... Now Iran's program is just not progressing as quickly as people assumed it would, because we've made it extremely difficult for them to get the materials they need."

Centrifuge explosions several years ago at Iran's main Natanz enrichment facility have been attributed to sabotage, as has the blocking of key components from places like Ukraine. The last IAEA report on Iran in February noted that of 8,610 centrifuges installed at Natanz – in a facility designed a decade ago to house more than 50,000 – only 3,772 were actually in use.

Iran has also complained in the past year that some of its nuclear scientists have "disappeared" during trips abroad, and charged that they ended up in US custody.

"When you look at efforts like that," says Mr. Levi, author of the 2007 book "On Nuclear Terrorism," "you can't just look at the successes, but at what Iran might not be doing.... When you think that your scientists are going to be bought off for information, you keep information much more closely, and you can't run your program as effectively." Levi says Iran has still made "significant advances" in recent years and that it remains an "open question" about whether Iran will go for a weapon.

"It's difficult to muster the kind of sanctions that drive changes in strategy on the part of Iran," Levi says. "[But] anything that we do that makes Iran run its programs based on political considerations … based on worries about sabotage and infiltration, makes them less able to run it as a proper technical engineering program."

Progress is slowed; can it be stopped?

But slowing down a nuclear program is not the same as stopping progress. Iran argues that under Article IV of the NPT, it has a right – like all signatories – to the entire nuclear fuel cycle and all nuclear technology required for a peaceful program to produce energy.

That provision has been recognized for decades as a loophole in the NPT, which enables a nation to legally reach the brink of technical ability for a bomb – under the guise of a peaceful program – then "break out" of the treaty. North Korea did exactly that in 2003.

To prevent Iran from doing the same, key Western powers, led by the United States, argue that doubts about Iran's nuclear intentions forestall the Islamic Republic's right to the full fuel cycle.

"There is really nothing you can do if the sanctions don't work, other than to use force," says Mr. Chubin, author of "Iran's Nuclear Ambitions" in 2006. "And since everybody agrees that … would be counterproductive – because it would drive [Iran's] program underground, reinforce a sense of victimhood, unite the regime, and so forth – there is really nothing you can do."